As a result of stricter environmental regulations worldwide, hydrogen is progressively becoming a very important clean energy source for both mobile and stationary applications. For hydrogen to replace fossil fuels as the fuel of choice for mobile applications, it requires the creation of a production and delivery infrastructure equivalent to those that currently exist for fossil fuels. As an alternative and an interim step toward the new hydrogen economy, various groups are currently investigating hydrocarbon steam reforming for onboard generation of hydrogen for use in fuel cell-powered vehicles, or for on-site production, in place of compressed or liquid hydrogen gas storage for stationary power generation applications (Choi and Stenger (2003) J. Power Sources 124:432, Darwish et al. (2003) Fuel 83:409, Liu et al. (2002) J. Power Sources 111:83, and Semelsberger et al. (2004) Int. J. Hydrogen Energy 29:1047). Methane steam reforming is currently attracting renewed interest in this regard, particularly for distributed power generation through the use of fuel cells. The process is widely practiced for large-scale hydrogen production and involves reacting steam with methane, through the endothermic and reversible methane steam reforming reaction, over supported nickel catalysts in packed bed reactors (reformers). Traditionally, these reformers have generally operated at temperatures often in excess of 1,000 K and pressures as high as 30 bar and reach relatively low equilibrium conversions (Elnashaie et al. (1990) Chem. Eng. Sci. 45:491, Xu and Froment (1989) AIChE J. 35:88, and Han and Harrison (1994) Chem. Eng. Sci. 49:5875). Such conditions are often neither convenient nor economical to attain for small-scale, on-site (or onboard) hydrogen generation. As a result, there is much current interest in the development of more effective reforming technologies.
Reactive separation processes have been attracting renewed interest for application in catalytic steam reforming. They include packed bed catalytic membrane reactors (MRs) (Hwang (2001) Korean J. Chem. Eng. 18:775, Lim et al. (2002) Chem. Eng. Sci. 57:4933, Park et al. (1998) Ind. Eng. Chem. Res. 37:1276, Nam et al. (2000) Korean J. Chem. Eng. 17:288, Saracco and Specchia (1994) Catal. Rev.-Sci. Eng. 36:305, and Sanchez and Tsotsis (2002) Catalytic Membranes and Membrane Reactors, Wiley-VCH, Weinheim, Germany) and, more recently, absorptive reactor (AR) processes (Xiu et al. (2004) Chem. Eng. Res. Des. 82:192, Xiu et al. (2003) Chem. Eng. J. (Amsterdam, Neth.) 95:83, Xiu et al. (2003) Chem. Eng. Sci. 58:3425, Xiu et al. (2002) AIChE J. 48:817, Xiu et al. (2002) Chem. Eng. Sci. 57:3893, Lee et al. (2004) Chem. Eng. Sci. 59:931, Ding and Alpay (2000) Chem. Eng. Sci. 55:3929, Ortiz and Harrison (2001) Ind. Eng. Chem. Res. 40:5102, Balasubramanian et al. (1999) Chem. Eng. Sci. 54:3543, Waldron et al. (2001) AIChE J. 47:1477, and Hufton et al. (1999) AIChE J. 45:248). Their potential advantages over the more conventional reformers have been widely discussed. They include (i) increasing the reactant conversion and product yield, through shifting of the equilibrium toward the products, potentially allowing operation under milder conditions (e.g., lower temperatures and pressures and reduced steam consumption), and (ii) reducing the downstream purification requirements by in situ separating from the reaction mixture the desired product hydrogen (in the case of MRs) or the undesired product CO2 (in the case of ARs).
MRs show substantial promise in this area and, typically, utilize nanoporous inorganic or metallic Pd or Pd alloy membranes (Sanchez and Tsotsis (2002) Catalytic Membranes and Membrane Reactors, Wiley-VCH, Weinheim, Germany). The latter are better suited for pure hydrogen production. However, metallic membranes are very expensive and become brittle during reactor operation (Nam et al. (2000) Korean J. Chem. Eng. 17:288) or deactivate in the presence of sulfur or coke. Nanoporous membranes are better suited for the steam reforming environment. They are difficult to manufacture, however, without cracks and pinholes and, as a result, often have inferior product yield. In addition, the hydrogen product in the permeate side contains substantial amounts of other byproducts, particularly CO2, and may require further treatment for use in fuel cell-powered vehicles.
Adsorptive reactors also show good potential (Xiu et al. (2004) Chem. Eng. Res. Des. 82:192, Xiu et al. (2003) Chem. Eng. J. (Amsterdam, Neth.) 95:83, Xiu et al. (2003) Chem. Eng. Sci. 58:3425, Xiu et al. (2002) AIChE J. 48:817, and Xiu et al. (2002) Chem. Eng. Sci. 57:3893). The challenge here, however, is in matching the adsorbent properties with those of the catalytic system. Two types of adsorbents have been suggested: potassium-promoted layered double hydroxides (LDHs), which operate stably only at lower temperatures (less than 500° C. (Waldron et al. (2001) AIChE J. 47:1477, Hufton et al. (1999) AIChE J. 45:248, and Ding and Alpay (2000) Chem. Eng. Sci. 55:3461), and CaO or commercial dolomite, which can be utilized at the typical steam reforming temperatures of 650-700° C. (Lee et al. (2004) Chem. Eng. Sci. 59:931) but requires temperatures higher than 850° C. for regeneration (Ortiz and Harrison (2001) Ind. Eng. Chem. Res. 40:5102 and Balasubramanian et al. (1999) Chem. Eng. Sci. 54:3543). These are very harsh conditions that result in gradual deterioration of the adsorbent properties and potentially sintering of the reforming catalyst (Ortiz and Harrison (2001) Ind. Eng. Chem. Res. 40:5102 and Balasubramanian et al. (1999) Chem. Eng. Sci. 54:3543). The mismatch between the reaction and regeneration conditions is likely to result in significant process complications.
Conventional steam reforming, particularly for methane (CH4), has been studied extensively and practiced routinely in the industry using a packed bed catalytic reactor (PBR). A high reaction temperature is required (i.e., >800° C. for CH4) to deliver a sufficient reaction rate and to overcome the equilibrium conversion limitations; this introduces an unfavorable environment for the exothermic water-gas-shift (WGS) reaction step. As a result, significant CO is present in the final product, which requires further conversion to H2 in a separate two-stage WGS reactor and additional post-treatment reactors (such as a partial oxidizer and a methanizer) to reduce the CO levels to meet the proton exchange membrane (PEM) feedstock specifications. This multiple-step reforming process adds significant process complexity, and is undesirable, particularly for small-scale distributed-type applications. MR technology (Sanchez Marcano and Tsotsis (2002) Catalytic Membranes and Membrane Reactors, Wiley VCH), primarily Pd membrane-based, has been proposed to streamline the reforming process by, for instance, integrating the reforming and WGS reaction in a single step or via the use of a one-step WGS. However, this Pd membrane-based MR suffers the following disadvantages: potential coking on the Pd surface as a result of H2 removal even at the lowest operating temperature, e.g., >450° C.; and only incremental, not dramatic increases in overall conversion resulting from bulk H2 removal. Theoretically, a nearly 100% conversion can be achieved by completely removing the H2 from the reactor side; however, the partial pressure of H2 available for permeation is too low for this to be realized in practice.
In the case of natural gas (NG), the reforming reaction is typically modeled as reforming of methane (by far its major component), which consists of the following two reactions:CH4+H2OCO+3H2; ΔH=+206.2 kJ/mol  (1′)CO+H2OCO2+H2; ΔH=41.2 kJ/mol  (2′)
Reaction (1′) is endothermic and equilibrium limited, and is, therefore, practiced at higher temperatures (>800° C.). Unfortunately, the WGS reaction (2′) is exothermic and is highly unfavorable at higher temperatures. This then necessitates the need for utilizing a separate reactor system for carrying out the WGS step at lower temperatures. The WGS reactor system is typically a dual-reactor system consisting of a high temperature reactor operating at ∥400-450° C., followed by a low temperature reactor, which operates at ˜250-300° C. This, then, adds significant process complexity to the fuel processing section. Nevertheless, even with the separate WGS reactor being present, the product contains ˜0.5-1% CO, substantially higher than what is permissible, for example, for PEM fuels cells. To make the use of such fuel cells possible (CO at the tens of ppm level is detrimental to performance) for power generation and mobile applications, an additional processing step for CO removal, typically a partial oxidation step (POX), is required, which further adds to the processing complexity and costs.